With recent developments in portable electronic devices including mobile phones and notebook PCs and with the recent advent of practical electric vehicles, there has been increased demand for compact, lightweight and high-capacity rechargeable batteries. To address this demand, a rechargeable lithium-ion battery is now marketed as a high-capacity rechargeable battery. The rechargeable lithium-ion battery includes a lithium-containing transition metal oxide, such as LiCoO2, as a positive electrode forming material and a carbon-based material as a negative electrode active material. The rechargeable lithium-ion battery has high energy density and may be made compact and lightweight, and thus has been considered as an important power supply for portable electronic devices.
A currently available lithium battery typically has a positive semiconductor electrode with a positive electrode active material of poor electron conductivity. In order to provide sufficient conductivity of the electrode, a conducting agent and a binder are added to a positive electrode mixture. The conducting agent may be carbon black, acetylene black, Ketchen black and graphite.
Japanese Unexamined Patent Application Publication, First Publication No. 2000-208147 discloses a positive electrode structure in which fine carbon black powder is attached to surfaces of positive electrode active material particles and the space between the particles of the positive electrode active material is filled with natural graphite and carbon fibers. Japanese Unexamined Patent Application Publication, First Publication No. 2006-86116 discloses a positive electrode structure which includes a positive electrode active material and a carbon-based conducting agent. The carbon-based conducting agent contains nano-sized carbon fibers. Japanese Unexamined Patent Application Publication, First Publication No. 2004-220909 discloses a positive electrode structure in which the space between the particles of the positive electrode active material is filled with carbon nanofiber.
High-rate discharging performance of a battery may be improved by an increased content of a conducting agent in a positive electrode. An excessively high content of the conducting agent, however, may result in a reduced ratio of the lithium-containing transition metal oxide, which may in turn result in reduced discharge capacity. A dispersing agent is used to disperse carbon black, acetylene black, Ketchen black or graphite, as well as normal carbon nanofiber and a carbon nanotube in a positive electrode forming material. The dispersing agent, however, may decompose resulting in the disadvantageous generation of gas. In a known positive electrode structure, the space between particles of the positive electrode active material is filled with a carbon-based material used as a conducting agent, which structure may require a relatively large content of conducting agent.
The present invention has been devised to solve these problems in the structures and materials of the related art positive electrodes for batteries. The present invention provides, among others, a positive electrode forming material and components thereof for forming a positive electrode structure having improved conductivity while keeping enhanced positive electrode performance. The positive electrode structure may be fabricated by dispersing electrical conducting materials in a solution without a dispersing agent and forming a network conductive film on a particle surface of the positive electrode active material.